Heteronuclear 2D (¹H-¹³C) MAS NMR Resolves the Electronic Structure of Coordinated Histidines in Light-Harvesting Complex II: Assessment of Charge Transfer and Electronic Delocalization Effect

Abstract

In a recent MAS NMR study, two types of histidine residues in the light-harvesting complex II (LH2) of Rhodopseudomonas acidophila were resolved: Type 1 (neutral) and Type 2 (positively charged) (Alia et al. J. Am. Chem. Soc.). The isotropic ¹³C shifts of histidines coordinating to B850 BChl a are similar to fully positively charged histidine, while the ¹⁵N shift anisotropy shows a predominantly neutral character. In addition the possibility that the ring currents are quenched by overlap in the superstructure of the complete ring of 18 B850 molecules in the LH2 complex could not be excluded. In the present work, by using two-dimensional heteronuclear (¹H-¹³C) dipolar correlation spectroscopy with phase-modulated Lee–Goldburg homonuclear ¹H decoupling applied during the t₁ period, a clear and unambiguous assignment of the protons of histidine interacting with the magnesium of a BChl a molecule is obtained and a significant ring current effect from B850 on the coordinating histidine is resolved. Using the ring current shift on ¹H, we refine the ¹³C chemical shift assignment of the coordinating histidine and clearly distinguish the electronic structure of coordinating histidines from that of fully positively charged histidine. The DFT calculations corroborate that the coordinating histidines carry ∼0.2 electronic equivalent of positive charge in LH2. In addition, the data indicate that the ground state electronic structures of individual BChl a/His complexes is largely independent of supermolecular π interactions in the assembly of 18 B850 ring in LH2.